The objective of the study is to demonstrate that serotonin receptor agonists (1A and 1B), nerve growth factor (NGF) and vascular endothelial growth factor (VEGF) regulate (by an autocrine mechanism) the balance between proliferation/apoptosis in bile duct ligated (BDL) rats by calcium and cAMP dependent changes in the Src/ Ras/Raf/MAPK pathway. This hypothesis is based upon: 1. Cholangiocytes express the serotonin 1A and 1B, neurotrophin, and VEGF receptors, VEGFR-2 and VEGFR-3; 2. Cholangiocytes secrete serotonin, NGF and VEGF; 3. Cross-talk between intracellular calcium and adenylyl cyclase (regulated by serotonin, NGF and VEGF) exists in cholangiocytes; 4. Administration of serotonin 1A and 1B agonists to BDL rats decreases ductal mass and activates apoptosis; 5. Administration of anti-NGF antibody inhibits cholangiocyte growth and activates apoptosis, 6. Administration of anti-VEGF antibody to BDL rats blocks cholangiocyte proliferation and activates apoptosis, and 7. Serotonin, NGF and VEGF regulation of cholangiocyte growth is modulated by the calcium and cAMP dependent changes in the PKA/Src/Ras/Raf/ MAPK pathway. Consistent with the concept that VEGF (secreted by Cholangiocytes) stimulates ductal proliferation, ligation of the hepatic artery induces a decrease in VEGF expression and cholangiocyte proliferation and an increase in apoptosis, whereas administration of VEGF to rats with hepatic artery ligation prevents the effects of hepatic artery ligation on VEGF expression and cholangiocyte proliferation/loss. The applicant proposes: (i) To demonstrate that activation of serotonin type I receptors inhibits cholangiocyte proliferation in BDL rats through calcium-dependent inhibition of the cAMP/PKA/ Src/Ras/Raf/B-Raf/MAPK pathway; (ii) To evaluate the role and mechanisms of action by which nerve growth factor regulates cholangiocyte proliferation and fibrosis; and (iii) To demonstrate that cholangiocytes secrete VEGF and regulate their own ductal mass (by a balance between proliferation and apoptosis) in BDL rats through an autocrine mechanism by modulating the secretion of VEGF. The studies raise the possibility that modulation of cholangiocyte serotonin, NGF and VEGF secretion may represent a new therapeutic approach for manipulating cholangiocyte growth/loss in liver diseases.